Conservation of toner in prior xerographic systems takes the form of a draft mode or of overall density control. The assignee of this invention has sold commercially for several years laser printers which permit density control of an entire page among high, medium, and low under operator control. None of these densities is so light as to be perceived as a gray in final printing, and the purpose of selecting one may be both to minimize the use of toner and to produce a black density which is satisfactory, or even preferred, to the operator. The saving of toner is a stated advantage of the selection of less than highest density. The change in density is implemented by changing the light modulation for each bit of the data map, specifically by illuminating fewer of a set of contiguous vertical slices corresponding to a bit in the bit map of the image being formed.
In another commercial xerographic laser printer which is prior to this invention, a draft mode prints a dark outline of characters with a gray fill internal to the outline. It is also known to simply print gray characters in a draft mode. The manners of implementation of such modes could vary. The foregoing known prior laser printer appears to print the gray by printing full bits while not printing other full bits.
In certain non-xerographic implementations a draft mode is associated with faster speed as well as a change in print quality. In wire matrix printers, which employ columns of small wires to impact a marking material to form images, the wires have a limited recovery time, so eliminating dots in images permits higher speeds. Higher speeds, however, are not generally a result of toner conservation in laser printers since the maximum speed of laser printers are not limited by the complexity of the final bit image to be printed.
The speed of laser printers can be limited by the time for their central processing unit to define a final bit image to be printed. Thus, if a toner conservation mode were to require derivation of a new or more complex mapping of bits or definition of the function of bits, this would be a serious defect.
U.S. Pat. No. 5,317,679 to Ueda et al is for edge smoothing to remove stair steps in which emphasis of edges is avoided.